Intermittent or no AM is usually due to a bad antenna ground and/or bad coax cable. When the FM is not working is when you would likely have a bad tuner in the radio. The fix for this problem will most likely be to replace the entire antenna base, mast and coax cable. Given the age of the truck it would also be a good idea to either remove the fender liner for access or reach in through the access holes in the fender under the hood and clean the underside of the antenna mounting hole on the fender itself down to bare metal with a wire wheel or sandpaper so the new antenna will get a good ground. The antenna gets its ground from the metal base contacting the metal of the underside of the fender. Be sure to route the new coax cable away from other wire harnesses, electrical parts or power sources to avoid any radiated noise with the new antenna.
Best Buy may be able to replace the antenna, but it is pretty easy to DIY. Once you get the new base mounted to the fender just follow the old coax down to the grommet where it enters the cab. On a full size Chevy I seem to remember the grommet is kind of low on the passenger's side of the firewall below the HVAC box. From there it is super easy to route the coax to the radio. You don't even need any tools to do it. The dash panel just snaps off and the new radio should be mounted in a plastic installation kit that has a plastic snap on each side, just push those down and pull the radio/kit out of the dash and swap out the antenna connector.
Your new radio uses a larger standard Motorola style antenna plug. There will be an antenna cable adapter for the GM mini-antenna plug that Best Buy would have installed on your existing OEM coax, but you will not need that with a new antenna unless you are replacing the old antenna with a new OEM Chevy antenna with the smaller connector on the coax. Best Buy or Auto Zone or any good independent car audio store should have a good aftermarket replacement GM antenna on hand or they should be able to order one for you.